1. Field of the Invention
The present invention relates to a pump actuation motor for an automotive antilock brake system.
2. Description of the Related Art
Nowadays, many an automobile is equipped with an antilock braking system (hereinafter referred to as automotive ABS) for controlling braking torque to prevent wheels from locking at braking, which may cause a slip of wheels over the road surface or a loss of steering wheel operational performance.
The automotive ABS controls the braking torque so that wheel slip rate at the time of braking will be maintained within an appropriate range, whereby the automotive ABS is structured so as to output a maximum braking force at any time, even under a slippery circumstance where steering performance of the wheels is deteriorated, for instance, such as rain, snow or a gravel road.
To describe an example of the automotive ABS, brake fluid can circulate within a hydraulic pressure circuit formed between a brake master cylinder that is so provided as to connect with a brake pedal for an automobile and wheel cylinders that are so provided as to connect with wheel brakes. In addition, there is provided an actuator which controls the hydraulic pressure by opening and closing normally-open solenoid valves and normally-closed solenoid valves that are disposed on the hydraulic pressure circuit that extends from the brake master cylinders to the wheel cylinders. The actions of the actuator are controlled by an electronic control unit that uses a microcomputer. The electronic control unit receives output on the travelling state of an automobile from such as wheel speed sensors that are provided on the wheels respectively.
The electronic control unit controls the actuator with a command in any of these modes: a pressure increase mode where the braking torque.is increased by increasing the hydraulic pressure that is applied from the brake master cylinders to the wheel cylinders, a holding mode where the braking torque is held by preventing the hydraulic pressure transmission from the brake master cylinders to the wheel cylinders, and a pressure decrease mode where the braking torque is decreased by temporarily reserving the hydraulic pressure from the wheel cylinders at reservoirs (tanks) to prevent locking state at the wheels.
Among these modes, at a transition to the pressure decrease mode, it is required to return brake fluid reserved in the reservoirs to the side of the brake master cylinders, and return the reservoirs to the initial empty state. For this purpose, pumps are provided in the hydraulic pressure circuit that is connected to the reservoirs and the brake fluid is returned to the master cylinders through drive of such pumps by the motor.
In these years, reduction in the size and weight has been in demand for the automotive design and thus a similar design concept has been in demand for the automotive ABS. Especially, for a pump actuation motor that is disposed in the engine room in the automotive ABS, there has been a compactification demand.
As for the structure of the motor, an hydraulic pump driving apparatus has been proposed on JP-A-9-323643, for instance. In this hydraulic pump driving apparatus, bearing holders are so formed in a cup shape as to expand in an axial direction at center parts of end walls respectively on the side of a housing that stores the hydraulic pump and on the side of a motor casing (a yoke). A motor rotation shaft is supported by fitting a ball bearing into such bearing holders.
Alternatively, there is proposed an electric-powered pump motor assembly on JP-W-8-510314. In this electric-powered pump assembly, a barrel-shape bearing is so formed with a rivet at a bottom part of a motor housing (a yoke) as to support by fitting in a blind hole that is provided at one end of a rotation shaft of a motor.
In the oil hydraulic pump driving apparatus proposed in the above-described JP-A-9-323643, since the bearing holders are formed in the cup shape expanding in the axial direction respectively on the sides of the housing and the motor casing (yoke) that support the motor rotation shaft, compactification of the motor size in the axial direction is difficult. Additionally, as for the motor rotation shaft, since the eccentric shaft portion, which is formed in the vicinity of the shaft end on the housing side, is connected to an interlink mechanism that actuates the hydraulic pump, downsizing of the ball bearing on the side of the casing (the yoke) that supports the rotation shaft like a cantilever is limited for the reason of strength. Another problem is an increase in the quantity of parts with sealing material since a window is formed at the bearing holder on the casing (yoke) side.
In the electric-powered pump motor assembly shown in JP-W-8-510314, since the barrel-shape bearing is mounted on the motor housing (the yoke) as a separate part, the problems are not only that it was difficult to attain an accurate coaxiality of the housing (the yoke) and the bearing but also that it is necessary to seal the joint between the mounted part of the housing (the yoke) and the barrel-shape bearing and thus the quantity of the parts is increased.
Further, since the barrel-shape bearing is a slide bearing that needs oil replenishment, such as greasing, there is a fear that the oil exploded out of the bearing might stick to a power feeding part to thereby cause a power feed failure and abnormal wear and thus that the durability would be deteriorated.
Moreover, since some clearance (20 .mu.m through 30 .mu.m) between the motor rotation shaft and the barrel-shape bearing for mounting is required, still another problem of increase in the loudness of the working motor sound was caused by room around the rotation shaft.